The patterns of soil nitrogen stocks and C:N stoichiometry under impervious surfaces in China

Abstract. Accurate assessment of soil nitrogen (N) storage and carbon (C):N stoichiometry under impervious surface areas (ISAs) is key to understanding the impact of urbanization on soil health and the N cycle. Based on 888 soil profiles from 148 sampling sites in 41 cities across China, we estimated the country’s N stock (100 cm depth) in the ISA soil to be 96.88 Tg N with a mean N density (N_ISA) of 0.59±0.35 kg m^–2, which was significantly lower (at all depths) than the soil N density (N_PSA = 0.83±0.46 kg m^–2) under the reference permeable surface areas (PSAs). Both N_ISA and N_PSA were higher than the mean N density of natural soils in China. These findings indicate that urbanization did not cause soil N loss, but the conversion of PSA to ISA could reduce soil N by 29 %. In comparison with the PSA, the ISA had a lower soil organic carbon (SOC) to N ratio (SOC:N) of 10.33±2.62 and a significant C–N correlation, showing no signs of C–N decoupling as suggested by the previous studies that might have been misled by the extremely high total C:total N ratio in the ISA soil. Moreover, the ISA had smaller variances in the SOC:N ratio than did the PSA, indicating convergence of soil C:N stoichiometry due to ISA conversion. Unlike natural soil, the SOC:N_ISA was negatively correlated with temperature. Unlike the vertical pattern in natural permeable soils, whose N density declined faster in the upper soil layers than in the lower layers, N_ISA decreased linearly with depth. In the spatial map of China’s N_ISA, the highest N_ISA was found in the northeast and the lowest in the southeast, and the highest SOC:N ratio was found in the Yangtze River Delta. This study revealed the unique spatial patterns of soil N under the ISA in China, which could potentially improve our capacity to assess and model urban biogeochemical cycles.